Many industrial and scientific processes require the measurement and control of extremely high temperatures. For example, measurements of the temperatures of molten metals are essential to proper process control in the metal processing industry. Two of the most common instruments used to determine the temperatures of molten metals are the optical pyrometer and the disposable lance thermocouple. However, each of these devices has its disadvantages. The optical pyrometer is not as accurate as is desirable, and can only measure the surface temperature of the molten metal. The disposable lance thermocouple is inaccurate, does not permit continuous measurement of the temperature of the molten metal, and its use involves some safety problems for the person using it.
As a result of the shortcomings of the optical pyrometer and the disposable lance thermocouple, considerable effort has been expended in developing an immersion pyrometer which has a long-term continuous reading capability. In one type of immersion pyrometer, a thermocouple junction is encased in a tube made of a metal with a high melting temperature which is coated with a ceramic, such as Al.sub.2 O.sub.3 or a mixture of Al.sub.2 O.sub.3 and Cr.sub.2 O.sub.3 to protect the metal tube from the molten metal environment. However, in use, the ceramic layer tends to spall and permit molten metal to contact and corrode the metal substrate. The inner metal tube cannot withstand attack by slag and/or the molten metal, and the tube, together with the sensing element enclosed therein, is quickly destroyed. The sensing element, usually a noble metal thermocouple, is expensive and it is desirable to be able to reuse it many times. However, structures which have been designed to protect thermocouples typically make for a slow thermal response, and thus are undesirable for many purposes.
U.S. Pat. No. 4,721,533 describes a temperature sensing system in which a sheath which encloses a temperature-sensing element is protected by a casing which is in contact with high temperature molten media. The casing comprises a combination of refractory metal oxides and graphite, and provides a combination of good heat conductivity, mechanical durability at elevated temperatures, and resistance to corrosion and erosion. The preferred sheath for use with the refractory metal oxide-graphite casing is described in U.S. Pat. No. 4,721,533 as being a cermet-coated closed end molybdenum tube in which the cermet coating comprises a plurality of porous layers of an alumina-chrominia-molybdenum cermet. An uncoated molybdenum tube was also disclosed as being useful as a sheath, although this was not the preferred embodiment. An uncoated molybdenum tube could be used since the outer casing isolates the molybdenum from the corrosive molten metal. The theory set forth in U.S. Pat. No. 4,721,533, was that "the refractory oxide-graphite composition of the outer casing results in a well-controlled carburization of the surface of the molybdenum tube". This appeared to slow the further degradation of the tube, and yielded a "self-healing" surface layer which would permit a long life, as for example, 100 hours.